Abstract

We have characterized the vibrational predissociation (VP) of the Ne(2)Br(2) van der Waals complex using time- and frequency-resolved pump-probe spectroscopy. After exciting Br(2) within the complex to a vibrational level 16<or=nu(')<or=23 in the B state, we follow the flow of halogen vibrational energy to the van der Waals modes in real time by recording the time-dependent behavior of Ne(2)Br(2) (nu(')), the NeBr(2) (nu(')-m) intermediates, and the Br(2) (nu(')-n) products. For Ne(2)Br(2) (nu(')=16-18), the only intermediate observed is NeBr(2) (nu(')-1), and the majority of the final product is Br(2) (nu(')-2), indicating the dissociation happens via two sequential direct VP steps. We fit the time-dependent behavior of these species to a sequential mechanism and extracted time constants for each step. For higher nu(') levels, the results show that the dissociation occurs via multiple pathways. Product Br(2) from levels lower than (nu(')-2) becomes much more important, with products as low as (nu(')-5) being observed. For nu(')=21, we observe both NeBr(2) (nu(')-1) and (nu(')-2) intermediates. The intermediates have significantly different kinetics, with the decay rate of the (nu(')-1) transient being nearly twice that of the (nu(')-2) transient. Similarly, both Br(2) (nu(')-2) and (nu(')-3) are formed in almost equal amounts, but the (nu(')-2) product formation rate is faster than the (nu(')-3) rate. The broad vibrational product state distributions and multiple dissociation pathways indicate that intramolecular vibrational energy redistribution becomes increasingly important for nu(')>19. We also report vibrational product state distributions for direct excitation to NeBr(2) 16<or=nu(')<or=23. For NeBr(2), the dominant product channel is Br(2) (nu(')-1) for all initial nu(') studied, consistent with this complex dissociating primarily via direct VP.